What action should be taken if a mid-efficiency gas furnace's induced draft motor does not operate on a call for heat?

When troubleshooting a mid-efficiency gas furnace with an induced draft motor that is not operating on a call for heat, verifying control voltage at the circuit board is a crucial step. The control voltage is essential for the operation of the furnace's components, including the induced draft motor. This motor needs to start first to establish a safe venting path before ignition can occur.

If there is no control voltage reaching the circuit board, the induced draft motor will not receive the necessary signal to operate, which will prevent the entire heating process from commencing. Therefore, checking the control voltage helps determine if the issue lies within the electrical system, potentially indicating problems with the transformer, wiring, or the control board itself.

Understanding that the induced draft motor must work properly as part of the safety interlock system is vital. If safety devices, such as pressure switches, do not receive the correct signals, they will also prevent operation. Consequently, testing the control voltage is a more direct method to ascertain if the cause of the problem lies in the control system before moving on to other components. This methodical approach ensures that the technician can effectively diagnose the furnace’s operational issues.